In general, the allocation of income of multinational enterprises to several jurisdictions for corporate tax purposes follows the OECD standard of the arm?s length principle. In contrast to the theory of the multinational enterprise, delimination of income following this principle does not fit systematically to the theoretical perception of the multinational enterprise as an integrated business. We show that almost any transfer price for tax purposes except marginal-cost pricing disturbs the optimal production plan. Therefore, it is inadequate to fix arm?s length related transfer pricing schemes to value cross border activities for tax purposes. Since the EU-Commission favours a comprehensive approach to tax integrated businesses in Europe following the unitary pinciple, we specify conditions under which an efficient allocation of resources result.